1. Field of the Invention
The present invention generally relates to voltage regulators (VRs) and voltage regulator modules (VRMs) and, more particularly, to switching VRs and VRMs having broad bandwidth to accommodate requirements for rapid transient response and with low steady-state current ripple such as are increasingly demanded for powering current and foreseeable microprocessors and the like.
2. Description of the Prior Art
Most electronic devices are ultimately powered with a direct current (DC) voltage even though power is often initially obtained from an alternating current (AC) power distribution system. Many such devices also require close voltage regulation of the power supply to function properly and reliably even when power is provided from a DC voltage source such as batteries. Microprocessors and other digital processing circuits using integrated circuits with high speed clocks and/or of high integration density require particularly close regulation of voltage since high integration density has required reduction of size of increased numbers of closely spaced integrated circuit elements which has, in turn, required operation at lower voltages.
Operation at lower voltages also implies operation at higher currents. State-of-the-art microprocessor designs nominally operate at 0.8 V and 150 A or more and foreseeable designs will operate at even lower voltages and higher currents. Such devices also generally operate in several modes such as an operating mode and one or more stand-by and/or sleep states in order to save overall power consumption which implies extremely wide and rapid swings in current requirements which must be supplied with high efficiency and, generally, high current density and low cost of the voltage regulator module (VRM), as well.
To provide acceptably increased efficiency, a combination of switching and filtering is generally preferred. In such arrangements, duty cycle of high speed switching is controlled to regulate voltage while accommodating changing current requirements of a load. A high rate of change of current requirements of a load requires a wide control bandwidth which, as a practical matter is generally limited to about one-sixth of the switching frequency. VRs currently in use are commonly designed to operate at a 300 KHz switching frequency and have a typical control bandwidth of about 50 KHz. While increasing control bandwidth can, in theory, be accomplished by increasing switching frequency, as a practical matter, increasing either or both of the switching frequency or the control bandwidth is a difficult technical challenge for several reasons such as developing sufficiently high voltage regulation resolution within a short switching cycle and accommodation of voltage sampling constraints for control through feedback (which imposes the limit of about one-sixth of the switching frequency on the control bandwidth due to sample-and-hold circuit effects, particularly if adaptive voltage positioning (AVP) is required, as well as increased losses due to, for example, body diode conductance of switches. Meeting transient load requirements for current and foreseeable microprocessor design require filter storage capacitance of 30 capacitors of 100 μF each or eight capacitors of 560 μF each which is prohibitive and unacceptable in terms of both cost and footprint.
The practical limit on control bandwidth of one-sixth of the switching frequency can be overcome by coupling the two (or more) output inductors of a multi-phase power converter of the buck converter type which also can reduce the steady-state current ripple while maintaining the same transient response which also reduces conduction losses in the switches. In such an arrangement, provision of an air gap in the center leg of the coupled inductor core distributes the magnetic flux more evenly and can reduce the core losses in the center leg. A commercial coupling inductor structure has been developed based on this concept, as depicted in FIG. 1A.
However, the known commercial coupling structure has the disadvantage that, in order to make the inverse coupling between phases, the windings must be around the core legs. Further, the winding length must be greater than in the non-coupled inductor and one more copper trace is needed in the layout to connect to the switching node as shown in FIG. 1B, increasing copper losses due to the longer trace. Additionally, the core of the known commercial coupling inductor is not flexible or physically symmetrical to match the preferred power stage layout and the voltage regulator arrangement in preferred layouts for a motherboard of current processor designs. These unavoidable physical constraints cause severely unbalanced ripple currents of each phase, as shown in FIG. 2 for a four phase embodiment.
It has also been proposed to couple all phases through multiple two-phase coupled inductors or transformers. However, this approach is very complex and requires a large number of magnetic components to be provided at a high cost and thus is unacceptable.